This invention relates to a multi-beam scanning system with an exposure control device and more particularly, to an image-forming apparatus such as a laser printer performing a scanning exposure step by a multi-beam laser on a photosensitive medium and thereafter developing and fixing.
Digital printers of this type using a multi-beam laser scanner, are able to generate a plurality of beams, resulting in an increased printing speed.
In order to generate a plurality of beams, a plurality of light sources are used, which need to be controlled efficiently in order to correct the intensity of each beam. Alternatively a plurality of beams can be generated by one light source with the help of a beam splitter.
Prior art techniques use a light-monitoring element, a photodiode for each laserbeam. In this case, the power of each laserbeam, expressed in mW, is independently controlled in accordance with an output from each light-monitoring element.
U.S. Pat. No. 4 788 560 discloses a multi-beam scanning apparatus allowing to eliminate a drawback of the conventional apparatus, i.e. the need for providing as many sensors for measuring the optical power as there are scanning beams. One single light intensity sensor is provided, thereby realizing a simple arrangement and an inexpensive apparatus.
Prior art systems measure the laser beam intensities by means of intensity sensors, that need to react very fast and do not take into account the sensitivity changes of the drum or belt due to temperature fluctuations and aging. In other words, there remains a need to have a reliable system that assures an homogeneous exposure on the photosensitive member.
JP61249066 creates an invariably stable black density image by measuring the potential on a photoconductive recording medium which is exposed and discharged electrostatically. According to the measurement result of the potential detection sensor, the quantity of light of the single beam exposing device is controlled.
In JP 63303373 plural image exposing devices are used to discriminate color images.
Every image exposing device creates an electrostatic image of one specific colour. The intensity of each of the exposing devices is adjusted in order to get a colour balanced image. The result is a stable image, but each colour component is generated by one laser beam only.